Implantable cardiac pacemakers of varying degrees of sophistication and operational capability are well known in the art. Earlier pacemakers were simple by today's standards, typically being capable of pacing only in a single chamber of the patient's heart, and only at an asynchronous, fixed, and uninhibited pacing rate. Today, pacemakers are available which are capable of synchronous, inhibited pacing in both chambers, at a pacing rate which may be varied according to detected intrinsic cardiac activity or some other physiological indication of the patient's metabolic needs.
Pacemakers are most commonly operated in conjunction with one or more leads, for conveying cardiac stimulating pulses from the pacemaker to the patient's heart, and for conveying electrical cardiac signals from the heart to the pacemaker's sensing circuitry. At least two different types of pacemaker leads, unipolar and bipolar, are commonly known and used.
Unipolar leads have only a single electrode and a single electrical conductor therein. The electrode is disposed at or near the distal end of the lead, which is situated in some particular location in the patient's heart, for example at the apex of the heart in the right ventricle, in the atrial chamber, or in the coronary sinus. The single electrode and conductor of a unipolar lead are used both for sensing (that is, for conducting electrical cardiac signals from the heart to the pacemaker) and for pacing (that is, for delivering stimulating pulses from the pacemaker to the heart.
Bipolar leads have two electrodes and two electrically isolated conductors therein. Often, one electrode, called the "tip" electrode, is a conductive contact disposed at the distal end of the lead, while a second electrode, called the "ring" electrode, is a conductive ring disposed on the lead body some distance back from the distal end of the lead. One of the isolated conductors conducts signals between the pacemaker and the tip electrode, while the other conducts signals between the pacemaker and the ring electrode.
In the case of unipolar pacing and sensing, the electrically conductive pacemaker canister can serve as an indifferent electrode, with pacing and sensing signals being conducted between the lead electrode and the pacemaker canister. In bipolar pacing and sensing, it is not necessary to use the pacemaker canister as an electrode in the pacing or sensing circuit, since pacing and sensing can occur between the tip electrode and the ring electrode, rather than between the tip electrode and the pacemaker canister as in a unipolar configuration.
As pacemaker functionality has become increasingly sophisticated and complex, it has become ever more important for the physician to monitor and obtain information about the pacemaker's operation and the heart's responses to the pacing therapy. Accordingly, many pacemakers today are capable of transmitting, for example via radio-frequency telemetry, information about the pacemaker's current programmable parameter settings and the pacemaker's operational status. In addition, the telemetry system may be capable of transmitting a representation of the intracardiac electrogram (EGM). The electrical cardiac signal received on the pacemaker lead and provided to the pacemaker's sensing circuitry can also be applied to the telemetry system, and transmitted in either analog or digital form to an external receiver, where the intracardiac electrogram can be recorded and/or viewed on a strip chart or ECG monitor.
In order to verify or optimize operation of an implanted pacemaker, a physician must be able to determine, among other things, when a stimulating pulse has been delivered to a chamber of the heart, and whether the stimulating pulse possessed sufficient energy to evoke a response from that chamber of the heart (i.e., whether "capture" was achieved). Such determinations can be difficult to make, particularly with some of the more advanced dual chamber, rate-responsive pacemakers in which the pacing rate may vary from one cardiac cycle to the next, and in which stimulating pulses may or may not be delivered depending upon sensed intrinsic cardiac activity.
Often, a physician will use a conventional surface electrocardiogram (surface ECG) equipment to monitor cardiac and pacemaker functions. Obtaining a surface ECG usually requires a hospital visit, and can involve placement of a dozen or more skin electrodes. This can be uncomfortable, inconvenient, and expensive for the patient. In addition, cardiac signals are subject to attenuation and distortion when they pass through the patient's tissue to be received by surface electrodes, and this can complicate the interpretation of the signals and assessment of cardiac and pacemaker function. In some cases, the morphological aspects of an electrical cardiac signal that must be detected to accurately assess cardiac or pacemaker function are simply not revealed in the surface ECG. For example, in determining whether a pacemaker has achieved capture, the physician must look at electrical cardiac signals for evidence of an evoked cardiac response to a pacing stimulus. The electrical evidence of an evoked response is a subtle characteristic of the cardiac signal. Additionally, the stimulating pulse is often not visible in surface ECG tracings of bipolar lead configurations.
Intracardiac electrogram (EGM) signals, derived from the electrical cardiac signal on the pacemaker lead(s) and transmitted to an external programmer as described above, are also known to be useful in monitoring and verifying pacemaker operation. One perceived drawback to such intracardiac EGM signals, however, is that since the same lead is used for pacing and sensing, the high stimulating pulse voltage spike, after-potentials, and electrode-tissue polarizations render the intracardiac EGM system "blind" to the cardiac signal for a period of time immediately following the delivery of each stimulating pulse. Unfortunately, it is during this time period immediately following a stimulating pulse that is of most interest in determining whether capture has been achieved (i.e., whether there has been an evoked response).
Typically, in dual chamber pacing, atrial and ventricular EGM signals are detected using the same lead configuration (i.e., unipolar or bipolar) as used for pacing in the respective chambers. For example, if atrial bipolar pacing between the tip and ring electrodes on the atrial lead, the atrial tip and ring electrodes will also provide the inputs to the atrial sense amplifier.
With conventional surface ECG electrodes, the electrogram signal viewed by the physician on the ECG monitor or strip chart recorder represents a composite of the atrial and ventricular signals of the heart. In the inventors' experience, it has generally been found that physicians are generally more familiar with this type of ECG waveform than the separate atrial and ventricular signals provided from intracardiac electrodes. Since a surface ECG signal represents both the atrial and ventricular signals simultaneously, the physician can easily perceive the timing relationships between activity in the two chambers, the relative magnitudes of atrial and ventricular signals, and possibly evidence of an evoked cardiac response. With the separate atrial and ventricular signals provided from intracardiac electrodes, on the other hand, the physician must somehow view both signals at once, such as on a dual-trace ECG monitor or a dual-trace strip chart recorder, in order to ascertain information about the interaction or coordination of atrial and ventricular cardiac activity, and about the operation of the pacemaker. Also, intracardiac EGM signals are susceptible to the aforementioned problems with after-potentials and polarization, making detection of evoked responses difficult.
Thus, although intracardiac electrogram signals offer greater resolution (i.e., less distortion and attenuation of cardiac signals) than surface ECG signals, intracardiac pacing leads are not effective for all purposes, since the above-noted problems of after-potentials and electrode-tissue polarizations render the pacing lead "blind" to electrical cardiac activity immediately following delivery of a stimulating pulse from that lead.
The ability to detect capture in a pacemaker is extremely desirable, since delivering pacing pulses having energy far in excess of the patient's stimulation threshold is wasteful of the pacemaker's limited power supply (typically a battery). Accordingly, several different techniques for verifying capture and adjusting a pacemaker's stimulation pulse energy have been shown in the prior art.
For example, U.S. Pat. No. 3,757,792 issued to Mulier et al. on Sep. 11, 1973 and entitled "Automatic Threshold Compensating Demand Pacemaker" discloses a pacemaker in which the energy of each stimulating pulse is decreased by an incremental amount from the previous stimulating pulse. The pacemaker disclosed in the '792 patent employs three electrodes: a sensing electrode, a stimulating electrode, and a common electrode. According to the '792 patent, the common electrode must be of sufficient size to avoid after-potential and polarization problems. The pacemaker disclosed in the '792 patent monitors sensed activity during a 100-msec "window" following each stimulating pulse; if loss of capture is detected (i.e., if no intrinsic cardiac activity is detected in the 100-mSec post-stimulation time window), the pacemaker next succeeding stimulating pulse is increased in energy, but only by an amount sufficient to raise the stimulating pulse energy safely over the last stimulating pulse which did achieve capture.
U.S. Pat. No. 3,920,024 issued to Bowers on Nov. 18, 1975 and entitled "Threshold Tracking System and Method for Stimulating a Physiological System" appears to disclose a pacemaker which continuously or periodically performs a stimulation threshold test to determine the minimum energy of a stimulating pulse that will achieve capture. The stimulating pulse energy level is periodically readjusted to be very near the stimulation threshold. Intrinsic cardiac activity is monitored during a 100-mSec time window following delivery of a stimulating pulse. If capture is not achieved, one or more "back-up" pulses are delivered until an evoked response is achieved. The stimulating pulse energy level is then re-adjusted upward.
In U.S. Pat. No. 3,949,758 issued to Jirak on Apr. 13, 1976 and entitled "Automatic Threshold Following Cardiac Pacer" there appears to be disclosed a pacemaker in which the energy of each stimulating pulse is decreased from that of the previous stimulating pulse, until loss of capture is detected, whereupon the stimulating pulse energy is increased, and then the incremental decreases are resumed and the process repeated.
U.S. Pat. No. 4,055,189 issued to Auerbach et al. on Oct. 25, 1977 and entitled "Condition Monitoring Pacer", U.S. Pat. No. 4,088,139 issued to Auerbach on May 9, 1978 and entitled "Automatic Detection and Registration of Failure Condition in a Cardiac Pacer Monitoring System", U.S. Pat. No. 4,096,865 issued to Auerbach et al. on Jun. 27, 1978 and entitled "Method and Apparatus for Monitoring a Timed Failure Condition Relationship in a Cardiac Pacer", and U.S. Pat. No. 4,114,892 issued to Auerbach on Mar. 20, 1979 and entitled "Cardiac Pacer and Monitor System" are commonly-assigned patents which each appear to disclose a pacemaker having fast-recovery circuitry operable to eliminate after-potentials in a sense amplifier dedicated to monitoring capture immediately following delivery of a pacing pulse. According to the '189, '139, '865 and '892 specifications, the dedicated capture sense amplifier is thereby able to amplify any evoked response of the heart to the stimulating pulse. Cardiac signals from the pacing/sensing leads are sampled at appropriate times following delivery of a stimulating pulse, and the sampled values are compared with predetermined threshold values to determine whether an adequate evoked response has occurred. If an insufficient evoked response (or no evoked response) is detected more than a predetermined number of times in a row, the pacemaker responds by increasing the energy of stimulating pulses, and by generating a marker pulse that can be recorded on a surface ECG.
U.S. Pat. No. 4,114,627 issued to Lewyn et al. on Sep. 19, 1978 and entitled "Cardiac Pacer System and Method with Capture Verification Signal" discloses a pacemaker having circuitry that is said to remove electrode polarization energy from the input of the pacemaker's sense amplifier, so that sensing can resume very shortly (18-mSec, according to the '627 specification) after a stimulating pulse is delivered. Thus, the pacemaker described in the '627 patent is said to be capable of detecting capture even though only a single lead is used for both pacing and sensing functions.
U.S. Pat. No. 4,114,628 issued to Rizk on Sep. 19, 1978 and entitled "Demand Pacemaker With Self-Adjusting Threshold and Defibrillating Feature" appears to disclose a pacemaker having an electromechanical transducer adapted to detect a ventricular contraction following delivery of a stimulating pulse. The pacemaker circuitry increments a counter upon delivery of stimulating pulse. If a ventricular response to the stimulating pulse is detected by the electromechanical transducer during the refractory period after the stimulating pulse, the counter is decremented. If no response is detected, the counter is not decremented. The energy level of pacing pulses varies according to the count value of the counter; thus, if one or more stimulating pulses are delivered without a corresponding ventricular response, the energy level of subsequent pacing pulses will be increased in response to the positive, non-zero count value in the counter.
U.S. Pat. No. 4,228,803 issued to Rickards on Oct. 21, 1980 and entitled "Physiologically Adaptive Cardiac Pacemaker" apparently discloses a pacemaker having a first sense amplifier for detecting QRS complexes, and a second sense amplifier for detecting T-waves (i.e., a ventricular evoked response). According to the '803 specification, the pacemaker varies its base pacing rate in proportion to the time interval between a delivered stimulus and an evoked response (T-wave) thereto. If no evoked response is detected, the stimulating pulse energy is incrementally increased. The '803 specification further describes a magnet mode in which pacing stimuli are delivered at an asynchronous rate with maximum energy. When the magnet is removed, the pacemaker reduces the stimulating pulse energy in a sequence of steps in successive cycles, until a pacing pulse is delivered which does not evoke a response. Then, the pacing energy is increased one step.
U.S. Pat. No. 4,305,396 issued to Wittkampf et al. on Dec. 15, 1981 and entitled "Rate Adaptive Pacemaker and Method of Cardiac Pacing" relates to a pacemaker having so-called "polarization compensation circuitry" that is said to produce a compensation signal that is combined, via a differential adder, to the sense amplifier inputs. According to the '396 specification, this compensation signal is said to counter the effects of electrode polarization, so that QRS complexes and T-waves can be accurately sensed following delivery of a stimulating pulse. The '396 specification also describes an initial, positive-going excursion of each stimulating pulse, followed by the conventional negative-going stimulating pulse itself. The initial, positive-going portion of the stimulating pulse is said to further compensate for the effects of electrode polarization.
U.S. Pat. No. 4,674,508 issued to DeCote on Jun. 23, 1987 and entitled "Low-Power Consumption Cardiac Pacer Based on Automatic Verification of Evoked Contractions", U.S. Pat. No. 4,674,509 issued to DeCote, Jr. on Jun. 23, 1987 and entitled "System and Method for Detecting Evoked Cardiac Contractions", U.S. Pat. No. 4,708,142 issued to DeCote, Jr. on Nov. 24, 1987 and entitled Automatic Cardiac Capture Threshold Determination System and Method", and U.S. Pat. No. 4,729,376 issued to DeCote, Jr. on Mar. 8, 1988 and entitled "Cardiac Pacemaker and Method Providing Means for Periodically Determining Capture Threshold and Adjusting Pulse Output Level Accordingly" are commonly-assigned patents which relate to a capture determination scheme. According to these patents, a capture detect circuit is provided which is said to operate based on an assumption that if pacing pulses are applied to the heart in closely-spaced pairs, only one of the two pulses can possibly evoke a cardiac response. Furthermore, according to these patents, if the first pulse of the pair is delivered at an energy level known to exceed the patient's capture threshold, then this first pulse will evoke a response, and the second one of the pair will not. The capture verification circuit described in these patents digitizes the "recovery artifact" (i.e., the electrode polarization occurring after a pacing pulse) following the second pacing pulse of the pair. According to these specifications, the digitized recovery artifact will be the same for any pacing pulse which does not evoke a response. Thereafter, only single pacing pulses need to be delivered; the recovery artifact following each pacing pulse is then compared to the digitized recovery artifact from the non-capture pacing pulse. Only recovery artifacts from other pacing pulses that did not evoke a response will match the digitized recovery artifact; a pacing pulse which does evoke a response will differ from the digitized artifact, and this detection of this difference indicates that capture has been achieved.
U.S. Pat. No. 4,955,376 issued to Callaghan et al. on Sep. 11, 1990 and entitled "Pacemaker With Improved Automatic Output Regulation", U.S. Pat. No. 4,969,460 issued to Callaghan et al. on Nov. 13, 1990 and entitled "Pacemaker With Improved Automatic Output Regulation", U.S. Pat. No. 4,969,461 issued to Callaghan et al. on Nov. 13, 1990 and entitled "Pacemaker With Improved Automatic Output Regulation", U.S. Pat. No. 4,969,462 issued to Callaghan et al. on Nov. 13, 1990 and entitled "Pacemaker With Improved Automatic Output Regulation", U.S. Pat. No. 4,969,464 issued to Callaghan et al. on Nov. 13, 1990 and entitled "Pacemaker With Improved Automatic Output Regulation", and U.S. Pat. No. 4,969,467 issued to Callaghan et al. on Nov. 13, 1990 and entitled "Pacemaker With Improved Automatic Output Regulation" (hereinafter collectively referred to as the Callaghan et al. patents.) are commonly-assigned patents which relate to a pacemaker said to be capable of automatic capture verification and pacing threshold determination. According to the Callaghan et al. patents, a "charge dump" circuit is provided for discharging polarization potentials on the output capacitor and electrode immediately after delivery of each pacing pulse. Pacing is peformed between the tip electrode of a bipolar pacing/sensing lead and the pacemaker canister, serving as a common or indifferent electrode. Sensing of intrinsic cardiac actitivity is performed in conventional bipolar manner (i.e., between the tip and ring electrodes of the bipolar lead). Sensing of cardiac activity following delivery of a pacing pulse, on the other hand, is performed between the ring electrode and the pacemaker canister.
According to the Callaghan et al. patents, a capture detection circuit is coupled to the ring electrode and is activated during a 60-mSec time window following delivery of pacing pulse. Capture verification is normally performed every four cardiac cycles; if capture is not detected upon capture verification, the pacing rate is temporarily increased by 5-PPM, in order to determine whether the apparent loss of capture is actually the result of a fusion beat (i.e., a simultaneous instrinsic and paced event). If the possibility of a fusion beat is ruled out, the stimulating pulse energy in increased incrementally until capture is obtained (and the energy is further increased to provide a safety margin between the stimulation threshold and the pulse energy.
Notwithstanding the variety of prior art arrangements for enabling a pacemaker to verify capture, it is believed by the inventors that there has yet to be shown in the prior art a method for verifying capture that is well-suited to dual-chamber pacemakers.
For example, many of the above-noted prior art references relating to capture verification (e.g., Mulier et al. '792, Bowers '024, Jirak '758, Auerbach et al. '189, and '865, Auerbach '139 and '892, Lewyn et al. '627, Rizk '628, Rickards '803, Wittkampf et al. '396, DeCote '508, '509, '142, and '376, and Callaghan et al. '376, '460, '461, '462, '464, and '467) appear to address only single-chamber (specifically, ventricular) pacing and sensing.
It appears to the inventors that in the prior art, the problems with electrode polarization and after-potentials has been dealt with primarily in two ways: either additional circuitry is required to quickly counteract such after-potentials following a stimulation pulse, or an additional electrode or lead is required that is dedicated to the capture sensing function. The additional circuitry is considered undesirable, since it itself consumes some power, and increases the size and complexity of the pacemaker's circuitry. A lead dedicated to the capture sensing function is also considered undesirable, particularly with dual-chamber pacemakers which already require two leads. Lastly, none of the above methods is totally without problems as no successful commercial product has been developed based upon the above patents.